info_dialog("Success", data, $.jgrid.edit.bClose, {
                    buttons:[{
                    text:'Assign', id:"buttid",imgpath:"/js/jqGrid/themes/basic/images/row_edit.gif", onClick : function(){alert('test');}
                    }],
                    width : 'auto',
                    left : 200,
                    top : 150
                });

BTW, i see the source about jqgird.common.js,and i found "info_dialog" function, but i could not found any parameter to configure the button image. and  i click  delete button on the grid paging bar, the button on  info_dialog could see the button image.